1. Field of the Invention
This invention relates generally to the field of packaging, and more specifically to the field of transparent plastic containers that are fabricated from an extrudable PET material using an extrusion blowmolding process.
2. Description of the Related Technology
Many products that were previously packaged using glass containers are now being supplied in plastic containers. Polyethylene terephthalate (PET) thermoplastic resins are polyester materials that provide clarity and transparency that is comparable to glass. PET possesses the processing characteristics, chemical and solvent resistance and high strength and impact resistance that are required for packaging products such as juices, soft drinks and water. PET containers are lightweight, inexpensive, recyclable and can be economically manufactured in large quantities. They will not shatter and create potentially dangerous shards when dropped, as glass containers may.
PET containers have conventionally been manufactured using the stretch blow molding process. This involves the use of a pre-molded PET preform having a threaded portion and a closed distal end. The preform is first heated and then is longitudinally stretched and subsequently inflated within a mold cavity so that it assumes the desired final shape of the container. As the preform is inflated, it elongates and stretches, taking on the shape of the mold cavity. The polymer solidifies upon contacting the cooler surface of the mold, and the finished hollow container is subsequently ejected from the mold.
Another well-known process for fabricating plastic containers is the extrusion blow molding process, in which a continuously extruded hot plastic tube or parison is captured within a mold and inflated against the inner surfaces of the mold to form a container blank. In such systems, the mold is typically designed to travel at the speed at which the extruded parison is moving when it closes on the parison so that the process can operate on a continuous basis. There are several different types of extrusion blow molding machines, including shuttle molds that are designed to travel in a linear motion and extrusion blow molding wheels that travel in a rotary or circular motion.
Extrusion blow molding is typically used to form plastic containers, such as motor oil containers, from nontransparent materials such as polyolefin or polyethylene. In the past, it was not typical to use extrusion blow molding to fabricate PET containers, because no commercially available PET material provided the required melt strength for extrusion blowmolding in addition to being compatible with standard PET recycling processes. More recently, however, extrudable PET materials have been made commercially available that can be processed at temperatures and conditions similar to standard PET and that provide the required melt strength for extrusion blow molding. Such materials have higher melt temperatures than the polyethylene or polyolefin materials that are typically used with extrusion blowmolding. A limited number of PET containers that are fabricated using extrusion blow molding have now been commercially introduced.
Despite the advantages of plastic materials such as PET, glass containers are still prevalent for certain products, such as liquor bottles. One of the reasons for this is that containers such as liquor bottles require a substantial amount of column or top load strength so that the structural integrity of the container is not compromised when the containers are stacked in boxes or pallets and subjected to substantial vertical compressive forces. In many container designs, the neck and shoulder portions are the weakest link in terms of the column strength of the container. In order to minimize material costs, it is desirable to make the sidewall of these container portions, as with the rest of the container, as thin as possible. However, such lightweighting comes at the expense of container strength, and in particular column strength.
Known measures for enhancing column strength in stretch blow molded PET containers include the provision of such features as vertical ribs. However, the formation of such features may not be practical using an extrusion blow molding process. Moreover, such features may not be aesthetically compatible with the intended product. For example, consumers may expect a liquor bottle to resemble a glass liquor bottle and have substantially smooth neck and shoulder portions.
A need exists for a PET container that can be manufactured using an extrusion blow molding process that exhibits superior column strength, particularly in the neck and shoulder portions of the container, without compromising the aesthetic character of the container.